Configure capacity buffers

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Capacity buffers improve the responsiveness and reliability of critical workloads by proactively managing spare cluster capacity using a Kubernetes CapacityBuffer custom resource definition. Using a capacity buffer lets you explicitly define a specific amount of unused node capacity within your cluster. This reserved capacity helps to ensure that GKE provisions nodes ahead of time.

When a high-priority workload needs to scale up quickly, the new workload can use the empty capacity immediately without waiting for node provisioning. This minimizes latency and avoids resource contention during sudden spikes in demand.

This page provides three methods for configuring capacity buffers: a fixed replicas buffer, a percentage-based buffer, and a resource limits buffer.

Before you begin

Before you start, make sure that you have performed the following tasks:

• Enable the Google Kubernetes Engine API.
Enable Google Kubernetes Engine API
• If you want to use the Google Cloud CLI for this task, install and then initialize the gcloud CLI. If you previously installed the gcloud CLI, get the latest version by running the gcloud components update command. Earlier gcloud CLI versions might not support running the commands in this document.
  Note: For existing gcloud CLI installations, make sure to set the compute/region property. If you use primarily zonal clusters, set the compute/zone instead. By setting a default location, you can avoid errors in the gcloud CLI like the following: One of [--zone, --region] must be supplied: Please specify location. You might need to specify the location in certain commands if the location of your cluster differs from the default that you set.

• Create, or have access to a GKE cluster on version 1.35.2-gke.1842000 or later.
• (Optional, but recommended) Enable node auto-provisioning on your cluster.

Create prerequisite Kubernetes objects

To configure a CapacityBuffer, you need a namespace that holds all of the required objects (the CapacityBuffer itself, and additional resources like a PodTemplate or workload). The PodTemplate and CapacityBuffer must be in the same namespace. You can create a namespace or use an existing namespace, including the default namespace.

Depending on which type of CapacityBuffer you're configuring, you also require one of the following:

• PodTemplate: defines the resource requirements for a single unit of buffer capacity. The configuration specified in the CapacityBuffer object references the Pod template.

• Workload: an existing workload that you reference in the CapacityBuffer object. This guide uses a Deployment object as an example workload, but capacity buffers support any of the following resource types:

  • Deployment
  • ReplicaSet
  • StatefulSet
  • ReplicationController
  • Job

  • CustomResourceDefinitions (CRDs) that implement the scale subresource.

    Note: If you are using a custom CRD, you might need to manually grant the cluster autoscaler permissions to read the scale subresource. See Permission errors for custom scalable resources.

This section provides examples of these objects. If you already have a workload that you want to configure with a capacity buffer, proceed to Apply a capacity buffer.

To create an example Kubernetes workload, complete the following steps:

1. Save the following manifest as namespace.yaml:

   apiVersion: v1
   kind: Namespace
   metadata:
     name: capacity-buffer-example
     labels:
       name: capacity-buffer-example
   

   This manifest creates a namespace called capacity-buffer-example.

2. Save the following manifest as buffer-pod-template.yaml:

   apiVersion: v1
   kind: PodTemplate
   metadata:
     name: buffer-unit-template
     namespace: capacity-buffer-example # the namespace must be the same namespace as the CapacityBuffer
   template:
     spec:
       terminationGracePeriodSeconds: 0
       containers:
       - name: buffer-container
         image: registry.k8s.io/pause:3.9
         resources:
           requests:
             cpu: "1"
             memory: "1Gi"
           limits:
             cpu: "1"
             memory: "1Gi"
   

   This manifest creates a PodTemplate that defines the resource requirements for a single unit of buffer capacity (1 CPU and 1Gi Memory). This configuration specifies the size of capacity units that GKE provisions for the buffer. For example, with this PodTemplate, GKE won't consider nodes with less than 1 CPU and 1Gi of available resources as part of the buffer, if the cluster scales up.

3. Save the following manifest as sample-workload-deployment.yaml:

   apiVersion: apps/v1
   kind: Deployment
   metadata:
     name: critical-workload-ref
     namespace: capacity-buffer-example # the namespace must be the same namespace as the CapacityBuffer
   spec:
     replicas: 10
     selector:
       matchLabels:
         app: critical-workload
     template:
       metadata:
         labels:
           app: critical-workload
       spec:
         containers:
         - name: busybox
           image: busybox
           command: ["sleep", "3600"]
           resources:
             requests:
               cpu: 100m
   

   This manifest creates a sample Deployment with 10 replicas, which is the reference object for the percentage-based buffer example in the next section.

4. Apply the manifests to your cluster:

   kubectl apply -f namespace.yaml -f buffer-pod-template.yaml -f sample-workload-deployment.yaml
   

5. Verify that GKE created the objects:

   kubectl get podtemplate -n capacity-buffer-example
   kubectl get deployment critical-workload-ref -n capacity-buffer-example
   

   The output is similar to the following:

   NAME                   AGE
   buffer-unit-template   1m
   
   NAME                    READY   UP-TO-DATE   AVAILABLE   AGE
   critical-workload-ref   10/10   10           10          1m
   

Apply a capacity buffer

This section provides examples of the different types of capacity buffers that you can apply to your workloads.

Note: It might take up to 5 minutes for the capacity buffer to react to changes.

Configure a fixed replicas buffer

Configuring a CapacityBuffer with fixed replicas specifies the exact number of buffer units that you want based on a PodTemplate.

To create a buffer with fixed replicas, complete the following steps:

1. Save the following manifest as cb-fixed-replicas.yaml:

   apiVersion: autoscaling.x-k8s.io/v1beta1
   kind: CapacityBuffer
   metadata:
     name: fixed-replica-buffer
     namespace: NAMESPACE
   spec:
     podTemplateRef:
       name: POD_TEMPLATE
     replicas: 3
   

   Replace the following:

   • NAMESPACE: the name of your namespace, for example capacity-buffer-example.
   • POD_TEMPLATE: the PodTemplate that defines your resource requirements, for example buffer-unit-template.

   This manifest creates a CapacityBuffer resource that references a PodTemplate to request a specific number of buffer units.

2. Apply the manifest:

   kubectl apply -f cb-fixed-replicas.yaml
   

3. Confirm that GKE applied the capacity buffer:

   kubectl get capacitybuffer fixed-replica-buffer -n NAMESPACE
   

   The replicas field in the status should show 3, which reflects the number of replicas that you defined in the manifest. The STATUS field should show ReadyForProvisioning.

Configure a percentage-based buffer

Configuring a percentage-based buffer dynamically sizes the buffer based on a percentage of an existing scalable workload.

To create a percentage-based buffer, complete the following steps:

1. Save the following manifest as cb-percentage-based.yaml:

   apiVersion: autoscaling.x-k8s.io/v1beta1
   kind: CapacityBuffer
   metadata:
     name: percentage-buffer
     namespace: NAMESPACE
   spec:
     scalableRef:
       apiGroup: apps
       kind: Deployment
       name: SCALABLE_RESOURCE_NAME
     percentage: 20
   

   Replace the following:

   • NAMESPACE: the name of your namespace.
   • SCALABLE_RESOURCE_NAME: the name of your scalable resource, for example critical-workload-ref.

   This manifest creates a CapacityBuffer resource that requests a buffer size equivalent to 20% of the referenced resource's replicas. If you're using the Deployment example from the previous section, the replica value is set to 10.

   Note: You can set the value of the percentage field to greater than 100 if you require a buffer larger than the current size of the referenced workload.

2. Apply the manifest:

   kubectl apply -f cb-percentage-based.yaml
   

3. Confirm that GKE applied the capacity buffer:

   kubectl get capacitybuffer percentage-buffer -n NAMESPACE
   

   Check the CapacityBuffer status. The replicas field should show a value from the percentage calculation. If you're using the Deployment example from the previous section, you should see 2 buffer units, which is 20% of the 10 replicas defined in the Deployment.

4. Test the dynamic scaling by manually scaling the Deployment up to 20 replicas:

   kubectl scale deployment critical-workload-ref -n NAMESPACE --replicas=20
   

   The CapacityBuffer controller reacts and automatically scales the buffer to 4 replicas.

Configure a resource limits buffer

You can use the limits field to define a maximum number of resources that the buffer should consume, calculated based on the PodTemplate size.

To create a resource limits buffer, complete the following steps:

1. Save the following manifest as cb-resource-limits.yaml:

   apiVersion: autoscaling.x-k8s.io/v1beta1
   kind: CapacityBuffer
   metadata:
     name: resource-limit-buffer
     namespace: NAMESPACE
   spec:
     podTemplateRef:
       name: POD_TEMPLATE
     limits:
       cpu: "5"
       memory: "5Gi"
   

   Replace the following:

   • NAMESPACE: the name of your namespace, for example capacity-buffer-example.
   • POD_TEMPLATE: the PodTemplate that defines your resource requirements, for example buffer-unit-template.

   This manifest creates a CapacityBuffer resource with a total limit of 5 CPUs and 5 GiB Memory. If you're using the PodTemplate example from the previous step, you define each unit as 1 CPU and 1Gi Memory, which should result in 5 buffer units.

2. Apply the manifest:

   kubectl apply -f cb-resource-limits.yaml
   

3. Confirm that GKE applied the capacity buffer:

   kubectl get capacitybuffer resource-limit-buffer -n NAMESPACE
   

   Check the CapacityBuffer status. The replicas field should show a value derived from the limits that you defined. If you're using the PodTemplate example from the previous section, you should see 5 buffer units because this is the maximum number of units that fit within the defined limits.

Remove capacity buffers

If you no longer need a capacity buffer for your workloads, delete the CapacityBuffer object. This removes the placeholder Pods and allows the cluster autoscaler to scale down the nodes.

kubectl delete capacitybuffer CAPACITY_BUFFER_NAME -n NAMESPACE

Replace CAPACITY_BUFFER_NAME with the name of the CapacityBuffer that you want to delete.

Troubleshooting

The following section contains information on resolving common issues with capacity buffers.

Capacity buffer not ready due to billing model

If you create a CapacityBuffer for a workload that uses the Pod-based billing model (pay-per-Pod), the capacity buffer won't be ready for provisioning.

To identify this issue, check the CapacityBuffer status:

kubectl describe capacitybuffer BUFFER_NAME -n NAMESPACE

Look for a condition of the type ReadyForProvisioning with a status of False.

To resolve this issue, ensure that your CapacityBuffer references a workload or PodTemplate that is compatible with node-based billing.

Permission errors for custom scalable resources

If you configure a CapacityBuffer to work with custom scalable objects (using the scalableRef field), the cluster autoscaler might fail to scale the buffer if it lacks the necessary permissions.

To resolve this issue, manually grant the required permissions by creating a ClusterRole and ClusterRoleBinding, such as in the following example:

apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: custom-scale-getter
rules:
- apiGroups: ["api.example.com"]
  resources: ["customreplicatedresources/scale"]
  verbs: ["get"]
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: ca-custom-scale-getter
subjects:
- kind: User
  name: "system:cluster-autoscaler"
  namespace: kube-system
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: custom-scale-getter

For more information about configuring RBAC, see the Kubernetes RBAC documentation.

What's next

• Learn more about capacity buffers.
• Refer to the CapacityBuffer CRD documentation.